The present embodiments relate to a local coil arrangement for magnetic resonance purposes.
A type of local coil arrangement is known, for example, from DE 10 2008 048 291 A1 or corresponding US 2010/099978 A1.
Imaging in magnetic resonance tomography is based on the spins of atomic nuclei aligned in a static basic magnetic field. For many applications, the homogeneity of the basic magnetic field is of significance for the image quality and also for the spatial registration of the images (e.g., distortion). In addition, the use of fat saturation techniques is relevant for numerous imaging techniques. With fat saturation techniques, the fatty tissue that emits a strong signal in many types of contrast is masked out. The masking out of the fatty tissue is a factor for the diagnostic usefulness of the magnetic resonance images, since with many types of sequences, pathological tissue displays a similar or even identical contrast behavior to the contrast behavior of fat.
Various methods for fat saturation are known from the prior art (e.g., Dixon or spectral fat saturation). Spectral fat saturation and related techniques utilize the fact that, with the same basic magnetic field, fat and water have slightly different resonance frequencies (e.g., deviation of about 3 ppm). A strong, sufficiently narrow-band transmission pulse at the exact fat frequency is, therefore, able to suppress the signal from fat without exerting a negative influence on the imaging of the protons belonging to the water molecules. However, a factor for the operational reliability of all techniques based on the spectral separation of fat and water is the homogeneity of the basic magnetic field, since the Larmor frequency is determined by the product of the gyromagnetic ratio and the basic magnetic field. If the basic magnetic field varies with a similar order of magnitude as the chemical shift or to a greater degree, the fat resonances and water resonances overlap, so the spectra may no longer be differentiated from each other.
Supra-conductive magnetic systems from the prior art enable magnetic field homogeneity with deviations of 1 ppm and less to be achieved within a volume measuring approximately 30 cm to 40 cm in diameter and 50 cm in length. Problems with respect to the spectral separation of water and fat may, therefore, occur in primarily extreme regions of the anatomy (e.g., the shoulder) that, due to the lack of space in an examination tunnel of a magnetic resonance device, may not be positioned centrally.
The inhomogeneities introduced by the tissue of the actual patient are more critical than the pre-known and deterministic inhomogeneities of the basic magnetic field. This is because human tissue has a relative magnetic permeability that differs, even if only slightly, from 1.0. As a result, for example, the discontinuities from air to tissue and vice versa cause severe distortion of the basic magnetic field. The inhomogeneous distribution of, for example, water, air, bone, and fat in the human body also causes distortion of the basic magnetic field. Such patient-induced distortion is individual to each patient and hence may not be compensated in advance.
It is known from the article “Construction and optimization of local 3rd order passive shim system for human brain imaging at 4T MRI,” by M. L. Jayatilake et al., Proc. Intl. Soc. Mag. Reson. Med. 19 (2011), page 3785, how to compensate residual inhomogeneity of the basic magnetic field using a passive shim coil. The shim coil used has dimensions of about 36 cm in diameter and length.
It is known from the article “Multi-Coil Shimming of the Mouse Brain,” by C. Juchem et al., Proc. Intl. Soc. Mag. Reson. Med. 19 (2011), page 97, how to surround the object under examination with a plurality of active shim coils and to set the shim currents of the shim coils individually in order to at least partially compensate the residual inhomogeneity. A similar disclosure may be found in the article “Dynamic Multi-Coil Shimming of the Human Brain at 7 Tesla,” by C. Juchem et al., Proc. Intl. Soc. Mag. Reson. Med. 19 (2011), page 716.
It is known from the article “B0 Shimming in 3 T Bilateral Breast Imaging with Local Shim Coils,” by S-K. Lee et al., Proc. Intl. Soc. Mag. Reson. Med. 19 (2011), page 715, how to arrange a shim coil in a breast coil arrangement and to use the shim coil for the compensation of residual inhomogeneity of the basic magnetic field.
For example, in the case of air-tissue transitions, the distortion of the basic magnetic field by the patient is strong and very spatially localized. Due to the strong spatial localization, the established methods are not suitable for the homogenization of the basic magnetic field.
In the prior art, the shim coils integrated in the magnetic resonance system are used to compensate the spatial variations of the basic magnetic field individually for each patient (e.g., following a calibration measurement). However, the shim coils known from the prior art are only able to compensate large areas of inhomogeneity. In the prior art, homogenization cushions are used for strongly localized effects. The cushions are made of a weakly diamagnetic material and displace the discontinuity from the air-tissue boundary to the air-cushion boundary in order to defuse the problem. However, the cushions result in additional distance between local coil and the patient and hence reduce the signal-noise ratio. In addition, the cushions may not be adapted to the specific inhomogeneity of the patient. The positioning of the cushions in the medical workflow is perceived as a drawback. The action of the cushions is heavily dependent on precise positioning, and the cushions are heavy.